Viscosity Breaker for Polyacrylamide Friction Reducers

ABSTRACT

A well treating fluid useful in slickwater fracturing processes contains polyacrylamide friction reducer and a viscosity breaker capable of reducing the viscosity of the well treating fluid to about the viscosity of water at ambient temperatures of typical underground formations. The viscosity breaker is selected from the group consisting of hydrogen peroxide, calcium peroxide, magnesium peroxide, and zinc peroxide and is present in an amount above about 0.002% by weight.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/811,740 filed on Jun. 8, 2006, which is entitled “Viscosity Breakerfor Polyacrylamide Friction Reducers.”

BACKGROUND OF THE INVENTION

1. Field of Use

The invention relates to the use of polyacrylamide friction reducers andmore particularly to the use of selected peroxides to break down theviscosity of the polyacrylamide friction reducers when frictionreduction is no longer required.

2. Related Art

In the petroleum industry it is an increasingly common practice toperform a procedure known as a slickwater fracturing. This is a methodof stimulating the production of hydrocarbons from a well by pumpingwater at high rates into the well, thus creating a fracture in theproductive formation. Practical and cost considerations for thesetreatments require the use of materials to reduce pumping pressure byreducing the frictional drag of the water against the well tubulars.Polyacrylamide polymers are very widely used for this purpose.

One ongoing problem with the use of these polymers is their persistence.A large fraction of the polyacrylamide used in a slickwater treatmentfrequently stays within the producing formation. This has several illeffects on the desired goal of stimulating production: the polymerresidue may actually plug some of the permeability of the rock; it mayhinder recovery of the water used in the fracturing treatment; and itprovides a source of nitrogen that can support the growth of bacteria inthe well. Polyacrylamide that is not absorbed but that remains insolution in recovered water will also make disposal of that water moredifficult.

A number of strategies have been used to deal with this problem.Chatterji in U.S. Pat. No. 4,144,179 teaches the use of organicperoxides in connection with a reducing agent in order to break up awell treatment fluid containing an organic gelling agent. Dawson in U.S.Pat. No. 6,983,801 teaches the use of esters and/or crosslinkers inconnection with a breaking agent to break down the viscosity of ahydratable polymer in a well treating fluid. However, there is still aneed for a viscosity breaker for a well treatment fluid that contains apolyacrylamide friction reducer.

SUMMARY OF THE INVENTION

A well treating fluid useful in slickwater fracturing processes containspolyacrylamide friction reducer and a viscosity breaker capable ofreducing the viscosity of the well treating fluid to about the viscosityof water at ambient temperatures of typical underground formations. Theviscosity breaker is selected from the group consisting of hydrogenperoxide, calcium peroxide, magnesium peroxide, and zinc peroxide and ispresent in an amount above about 0.002% by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors have discovered that polyacrylamides of the type used inslickwater fracturing can be degraded to the point of having a viscosityabout that of water by agents selected from the group of hydrogenperoxide, calcium peroxide, magnesium peroxide, and zinc peroxide. Themetal peroxides may be encapsulated as taught in U.S. Pat. No.4,506,734. Loadings of as low as 0.002% by weight of solution of thesematerials has been found to destroy the viscosity of polyacrylamidesolutions within a few hours at room temperature without interferingwith the friction reduction properties required of the polymers withinthe minutes after mixing the agents. Only a very few minutes ofeffective friction reduction are required or even desirable inslickwater fracturing treatments. The agents of this invention areeffective without the addition of supplemental reducing agents and/orcrosslinkers.

In a preferred embodiment hydrogen peroxide is added at a concentrationof about 0.004%, to a well treating fluid containing fresh water andabout 0.01% polyacrylamide. For ease of handling the hydrogen peroxideis preferably added as a 7% solution and the polyacrylamide is added asabout a 32% emulsion. The well treating fluid may optionally containother additives such as surfactants, scale inhibitors, andmicrobiocides. However, the well treating fluid does not contain anywater soluble transition metal salts, esters, cross-linking compounds,or free radical scavengers. The viscosity of the above well treatingfluid, which has an initial viscosity of about 2-5 centipose, returns tothe approximate viscosity of water, which is about 0.79 centipose,within about 18 hours at a temperature of about 90° F.

Various embodiments of the current invention may be better understood byreference to the following examples. All viscosities were measured inthe form of flow times through a Cannon-Fenske viscometer, which is apiece of glassware in which a precisely repeatable volume of fluid isallowed to flow through a length of fine capillary tubing. The timetaken for this flow is recorded and compared with the time taken by afluid of known viscosity. The Cannon-Fenske viscometer was selectedbecause electromechanical instruments are much less accurate in theselow ranges. The testing was conducted at 90° F. at which the viscosityof water is 0.79 centipose. This corresponds to a time of 145 secondsthrough the particular Cannon-Fenske viscometer used here.

Example 1

A well treating fluid was produced by adding 0.33 gallons of thepolyacrylamide emulsion sold as “PLEXSLICK 961,” which is manufacturedby Cytec Industries of North Carolina per 1000 gallons of fresh water.This corresponds with about 0.01% weight percent of polyacrylamide. Forsample A, 1.0 gallon per 1000 gallons of the 7% hydrogen peroxidesolution disclosed above was added immediately to the well treatingfluid prior to the start of testing. This corresponds to about 0.007% byweight of hydrogen peroxide in the well treating fluid. For Sample B,0.5 gallons per thousand gallons of the 7% hydrogen peroxide solutionwas added immediately to the well treating fluid prior to the start oftesting. This corresponds to about 0.0035% by weight of hydrogenperoxide in the well treating fluid. For Sample C, 0.33 gallons of the7% hydrogen peroxide solution was added immediately to the well treatingfluid prior to the start of testing. This corresponds to about 0.002% byweight of hydrogen peroxide in the well treating fluid. For each sample,the fresh solution was transferred to the viscometer and its viscositywas measured at the intervals noted in the table. The viscosities arereported in the number of seconds it took to pass through aCannon-Fenske viscometer. Times through this Cannon-Fenske viscometercan be converted to centipose by multiplying the time by 0.00545.

TABLE 1 Time Sample A Sample B Sample C Initial 186 183 185 1 hour  175172 176 2 hours 170 166 166 4 hours 165 160 166 6 hours 160 155 160 18hours  146 149 153

Example 2

A well treating fluid was produced according to the method described in

Example 1 except that the 0.5 gallons of the polyacrylamide emulsionsold as “PLEXSLICK 961,” which is also manufactured by Cytec Industrieswas added per 1000 gallons of fresh water. This corresponds with about0.0175% by weight of polyacrylamide. For sample D, 1.0 gallons of the 7%hydrogen peroxide solution was added immediately to the well treatingfluid prior to the start of testing. This corresponds to about 0.007% byweight of hydrogen peroxide in the well treating fluid. For Sample E,0.5 gallons of the 7% hydrogen peroxide solution was added immediatelyto the well treating fluid prior to the start of testing. Thiscorresponds to about 0.035% by weight of hydrogen peroxide in the welltreating fluid. For Sample F, 0.33 gallons of the 7% hydrogen peroxidesolution was added immediately to the well treating fluid prior to thestart of testing. This corresponds to about 0.002% by weight of hydrogenperoxide in the well treating fluid.

TABLE 2 Time Sample D Sample E Sample F Initial 213 215 215 1 hour  183196 202 2 hours 172 188 193 4 hours 159 181 185 6 hours 152 176 180 18hours  149 165 173

Example 3

A well treating fluid was produced according to the method described in

Example 1 except that the 0.33 gallons of the cationic polyacrylamideemulsion sold as “PLEXSLICK 982C,” which is manufactured by CytecIndustries was added per 1000 gallons of fresh water. This correspondswith about 0.01% by weight of polyacrylamide. For sample G, 1.0 gallonsof the 7% hydrogen peroxide solution was added immediately to the welltreating fluid prior to the start of testing. This corresponds to about0.007% by weight of hydrogen peroxide in the well treating fluid.

TABLE 3 Time Sample G Initial 174 1 hour  169 2 hours 161 4 hours 152 6hours 147 18 hours  145

Example 4

A well treating fluid was produced according to the method described in

Example 1 except that the 0.5 gallons of the cationic polyacrylamideemulsion sold as “PLEXSLICK 982C,” which is manufactured by CytecIndustries was added per 1000 gallons of deionized water. Thiscorresponds with about 0.0175% by weight of polyacrylamide. For sampleH, 1.0 gallons of the 7% hydrogen peroxide solution was addedimmediately to the well treating fluid prior to the start of testing.This corresponds to about 0.007% by weight of hydrogen peroxide in thewell treating fluid.

TABLE 4 Time Sample H Initial 1 hour  192 2 hours 185 4 hours 176 6hours 151 18 hours  147

As can be seen from the above examples, hydrogen peroxide in amountsranging from about 0.002 to about 0.007 weight percent are effective atreducing the viscosity of a well treating fluid containing from about0.01 to about 0.02 weight percent of two polyacrylamide emulsions thatare commonly used in slickwater fracturing to about the viscosity ofwater in about 18 hours at 90° F.

The above descriptions of certain embodiments are made for the purposesof illustration only and are not intended to be limiting in any manner.Other alterations and modifications of the preferred embodiments willbecome apparent to those of ordinary skill in the art upon reading thedisclosure, and it is intended that the scope of the invention disclosedherein be limited only by the broadest interpretation of the appendedclaims to which the inventors are legally entitled.

1. A well treating fluid for use in slickwater fracturing comprising:water; a polyacrylamide friction reducer present in an amount of fromabout 0.005% to about 0.04%; and a viscosity breaker present in anamount above about 0.002% by weight, wherein the viscosity breaker is aperoxide selected from the group consisting of hydrogen peroxide,calcium peroxide, magnesium peroxide, and zinc peroxide; whereby thewell treating fluid has a viscosity and the viscosity is reduced from aninitial viscosity to about the viscosity of water within about 18 hours.2. The well treating fluid of claim 1 wherein the viscosity breaker ishydrogen peroxide.
 3. The well treating fluid of claim 1 wherein thepolyacrylamide is cationic.
 4. The well treating fluid of claim 1wherein the water is fresh water.
 5. The well treating fluid of claim 1wherein the water is salt water containing less than about 10% saltcontent.
 6. The well treating fluid of claim 1 further comprising otheradditives selected from the group consisting of surfactants, scaleinhibitors, microbiocides, and mixtures thereof.
 7. The well treatingfluid of claim 1 wherein the well treating fluid has an initialviscosity below about 3 centipoise at a temperature of about 90° F. 8.The well treating fluid of claim 2 wherein: the polyacrylamide frictionreducer is present in an amount of from about 0.005% to about 0.04% byweight; the viscosity breaker is present in an amount of from about0.002% to about 0.02% by weight; and the well treating fluid has aninitial viscosity from about 0.8 to about 5 centipoise at a temperatureof about 90° F.
 9. The well treating fluid of claim 1 wherein theviscosity breaker is an encapsulated peroxide selected from the groupconsisting of calcium peroxide, magnesium peroxide and zinc peroxide.10-21. (canceled)